Mice exposed for 20 min daily to aerosolized ovalbumin (OVA) for 10 d at concentrations from 1 to 0.01% OVA made greatly reduced immunoglobulin (Ig)-E responses to subsequent immunogenic OVA challenges, given either intraperitoneally or by aerosol. This IgE-specific unresponsiveness lasted for at least four months. However, these aerosol-treated mice were primed for larger OVA-specific IgG1 and IgG2a responses. The specific reduction in IgE responses was not due to preferential induction of a T helper (Th)-1 response as aerosol OVA-primed mice made greatly reduced Th2 and no detectable Th1 response after rechallenge in vitro. Consistent with this, the increase in circulating eosinophils observed in control Th2-primed mice was absent in aerosol OVA-treated animals. Interferon (IFN)-γ was not required for this unresponsiveness, as IFN-γ knockout mice and anti-IFN-γ antibody-treated wild-type mice had greatly reduced levels of IgE similar to wild-type controls. CD8+ T cells played a relatively small role as IgE responses were reduced to about the same extent in β2 microglobulin-deficient, or in anti-CD8-treated wild-type mice as in normal mice after aerosol OVA treatment. Similarly, T cell receptor (TCR)-γ/δ T cells were not required for maximal inhibition of the IgE response. These results demonstrate that exposure to inhaled protein antigens can induce a state of unresponsiveness of CD4+ T cells that results in a prolonged loss of IgE and eosinophil responses to subsequent challenges. This T cell unresponsiveness was shown not to require CD8+ or TCR-γ/δ+ T cells or IFN-γ.